After what Fingers said about me in the 'can someone explain to me' thread in the general car chat forum, I am now somewhat vary about saying anything
.......Hang on, no I'm not:
Be aware that lowering a car changes the angle between the suspension and steering rack (carefully designed by the car manufacturer). This can introduce 'bump steer' (the wheel turns in and out when the suspension moves up and down) making the car jittery on anything other than very smooth surfaces.
You also change the angle of the suspension with the road surface when lowering a car by simply reducing spring length. This can have detrimental effects on handling, although this may be masked by the stiffer suspension 'feeling better'.
Having said all this, lowering by 30mm car shouldn't have too detrimental an effect on a road car, but watch out for increased or uneven tyre wear.
If you are serious about improving your car's handling as opposed to just doing what everyone else does (often badly), go to a library and order 'HOW TO BUILD AND MODIFY SPORTSCAR AND KIT CAR SUSPENSION AND BRAKES FOR ROAD AND TRACK' by Des Hammill.